main is red: two pcapng crawlers emit a self-recursive _missing_ body, causing RecursionError

[JarryShaw]

main is red at e58618bdf: 12 tests fail across three files — tests/const/test_const_enum_lookup.py, tests/const/test_const_enum_no_mint.py and tests/protocols/misc/test_pcapng_unit.py (7 of the 12) — surfacing as five Python 3.10–3.14 legs plus Gate and Required checks passed. One defect, all RecursionError.

tests/const/test_const_enum_lookup.py:216: AssertionError
  pcapkit.const.pcapng.secrets_type.SecretsType(0) unexpectedly raised
  RecursionError('maximum recursion depth exceeded')

Reproduced on 946b84e83 with the tree asserted; the cycle is three frames:

const/pcapng/secrets_type.py:42  _missing_        |  return cls(value)
aenum/_enum.py:1810              __call__         |  return cls.__new__(cls, value)
aenum/_enum.py:2266              __new__          |  result = cls._missing_value_(value)
aenum/_enum.py:2319              _missing_value_  |  return cls._missing_(value)   -> repeat

Root cause. pcapkit/vendor/pcapng/record_type.py and pcapkit/vendor/pcapng/secrets_type.py render a two-line _missing_ body whose first line does not return:

miss = [
    "cls._unregistered_member(value, 'Unassigned')",
    'return cls(value)'
]

That was harmless while the first line was extend_enum(...), which registers the member so the following cls(value) found it. #861 replaced it with _unregistered_member, which deliberately does not register — so cls(value) misses again and re-enters _missing_.

Why it was latent. #861 corrected the two generated files and not the two crawlers, so #861's own CI and 70fa92010 were genuinely green (59 ok / 0 fail). e58618bdf regenerated from the stale crawlers and reintroduced it. #861's cross-review verified byte-identical regeneration for 5 sampled crawlers out of 82; these two were not sampled.

Scope is exactly 2 of 103 registries — the other 101 carry return cls._unregistered_member(...). Fix and a crawler-layer guard in the PR that follows.